This trial is conducted in Japan. The aim of this clinical trial is to investigate the blood sugar lowering effect of NN5401 (insulin degludec/insulin aspart (IDegAsp)) in Japanese subjects with type 1 diabetes. Each subject will be randomised to one of the two possible treatment sequences (NN5401 followed by biphasic insulin aspart (BIAsp) 30 or biphasic insulin aspart 30 followed by NN5401).